1. Field of the Invention
The present invention relates to a measuring apparatus such as a surface plasmon resonance sensor for quantitatively analyzing a substance in a sample by utilizing the excitation of surface plasmon.
2. Description of the Related Art
In metals, if free electrons are caused to vibrate in a group, a compression wave called a plasma wave will be generated. The compression wave, generated in the metal surface and quantized, is called surface plasmon.
There have hitherto been proposed various kinds of surface plasmon resonance sensors for quantitatively analyzing a substance in a sample by taking advantage of a phenomenon that surface plasmon is exited by a light wave. Among such sensors, one employing a system called “Kretschmann configuration” is particularly well known (e.g., see Japanese Unexamined Patent Publication No. 6(1994)-167443).
The surface plasmon resonance sensor employing the “Kretschmann configuration” is equipped with a dielectric block formed, for example, into the shape of a prism; a metal film, formed on one surface of the dielectric block, for placing a sample thereon; and a light source for emitting a light beam. The surface plasmon resonance sensor is further equipped with an optical system for making the light beam enter the dielectric block so that a condition for total internal reflection (TIR) is satisfied at the interface between the dielectric block and the metal film and that various angles of incidence, including a surface plasmon resonance condition, are obtained; and photodetection means for measuring the intensity of the light beam satisfying TIR at the interface, and detecting surface plasmon resonance, that is, attenuated total reflection (ATR).
To obtain various angles of incidence in the aforementioned manner, a relatively thin light beam can be emitted so that it strikes the above-mentioned interface at different angles of incidence, or a relatively thick beam can be emitted so that it strikes the interface convergently or divergently. In the former, the light beam whose reflection angle varies with the incidence angle thereof can be detected by a small photodetector that is moved in synchronization with the reflection angle variation, or by an area sensor extending along a direction where the reflection angle varies. In the latter, on the other hand, the light beam can be detected by an area sensor extending in a direction where all the light beam components reflected at various angles are received.
In the surface plasmon resonance sensor mentioned above, an evanescent wave with electric field distribution is generated in a sample in contact with the metal film, if a light beam strikes the metal film at a specific incidence angle θsp greater than a critical incidence angle at which total internal reflection (TIR) takes place. The generated evanescent wave excites surface plasmon at the interface between the metal film and the sample. When the wave vector of the evanescent wave is equal to the wave number of the surface plasmon and therefore the wave numbers between the two are matched, the evanescent wave resonates with the surface plasmon and the light energy is transferred to the surface plasmon, whereby the intensity of the light satisfying TIR at the interface between the dielectric block and the metal film drops sharply. This sharp intensity drop is generally detected as a dark line by the above-mentioned photodetection means.
Note that the above-mentioned resonance occurs only when an incident light beam is a p-polarized light beam. Therefore, in order to make the resonance occur, it is necessary to make a p-polarized light beam strike the interface, or to detect only the p-polarized light component of an incident light beam.
If the wave number of the surface plasmon is found from the specific incidence angle θsp at which ATR takes place, the dielectric constant of a sample to be analyzed can be calculated by the following Equation:Ksp(ω)=(ω/c){εm(ω)εs}½/{εm(ω)+εs}½where Ksp represents the wave number of the surface plasmon, ω represents the angular frequency of the surface plasmon, c represents the speed of light in vacuum, and εm and εs represent the dielectric constants of the metal and the sample, respectively.
If the dielectric constant εs of a sample is found, the density of a specific substance in the sample is found based on a predetermined calibration curve, etc. As a result, the specific substance in the sample can be quantitatively analyzed by finding the specific incidence angle θsp at which the intensity of the reflected light at the interface drops sharply.
As a similar sensor making use of ATR, there is a leaky mode sensor (e.g., see “Spectral Researches,” Vol. 47, No. 1 (1998), pp. 21 to 23 and pp. 26 to 27). This leaky mode sensor is equipped with a dielectric block formed, for example, into the shape of a prism; a cladding layer formed on one surface of the dielectric block; and an optical waveguide layer, formed on the cladding layer, for placing a sample thereon. The leaky mode sensor is further equipped with a light source for emitting a light beam; an optical system for making the light beam enter the dielectric block at various angles of incidence so that a condition for total internal reflection (TIR) is satisfied at the interface between the dielectric block and the cladding layer and that ATR occurs by a waveguide mode excited in the optical waveguide layer; and photodetection means for measuring the intensity of the light beam totally reflected at the interface between the dielectric block and the cladding layer, and detecting the excited state of the waveguide mode, that is, ATR.
In the leaky mode sensor mentioned above, if a light beam strikes the cladding layer through the dielectric block at incidence angles greater than a critical incidence angle at which TIR takes place, the light beam is transmitted through the cladding layer and then only light with a specific wave number, incident at a specific incidence angle, propagates through the optical waveguide layer in a waveguide mode. If the waveguide mode is excited in this manner, the greater part of the incident light is confined within the optical waveguide layer, and consequently, ATR occurs in which the intensity of light totally reflected at the above-mentioned interface drops sharply. Since the wave number of the light propagating through the optical waveguide layer depends on the refractive index of a sample on the optical waveguide layer, both the refractive index of the sample and the properties of the sample related to the refractive index thereof can be analyzed by finding the above-mentioned specific incidence angle θsp at which ATR takes place.
Note that there are several types of measuring apparatuses utilizing TIR, such as the surface plasmon sensor or the leaky mode sensor, wherein light is made incident on an interface at an incidence angle in which conditions for TIR are obtained, and qualitative analysis is performed on a sample by measuring the change in the state of light totally reflected at the interface due to the evanescent waves generated by the light, other than those that measure the specific incidence angle at which ATR occurs. For example, there are those that make light beams of a plurality of wavelengths incident on an interface, and measure the degree of ATR for each wavelength, or those that divide a portion of a light beam made incident on an interface before the light beam enters the interface, and make the divided light beam interfere with the light beam reflected at the interface, and measure the state of said interference, etc.
In the conventional surface plasmon resonance sensor or leaky mode sensor of the type described above, when a single sample (the same measuring unit) is measured a plurality of times at predetermined time intervals in order to examine a change in the state thereof, there are cases where the sample and the dielectric body are both exchanged to efficiently measure a plurality of samples. In this case, if one sample is removed from the measuring apparatus and then the sample is again set, there is a disadvantage that difference (tilt) will occur between the first base line (aforementioned interface) and the next base line. If the tilt of the base line is a longitudinal tilt that changes the incidence angle of a light beam, the angle of the reflected light being measured will be shifted, resulting in a reduction in the measurement accuracy.
In addition, even when a sample is not exchanged, there are cases where the tilt of the base line changes slightly due to vibration, etc., when a table with a plurality of samples is being rotated. In such a case, the tilt of the base line during a plurality of measurements causes errors in measurement.
Furthermore, if the transverse tilt of the interface which shifts the angle of reflected light, as well as the longitudinal tilt of the interface which changes the incidence angle of the light beam, occurs, there are cases where the reflection direction of reflected light changes and therefore the reflected light cannot be received by the light-receiving surface of photodetection means. Thus, the longitudinal and transverse tilts of the interface result in a reduction in the accuracy of measurement.